Extreme ultraviolet lithography (EUVL, also known as soft x-ray projection lithography, and also abbreviated as EUV) is a contender to replace deep ultraviolet lithography for the manufacture of 14 nm, and smaller, minimum feature size semiconductor devices.
However, extreme ultraviolet light, which is generally in the 5 to 40 nanometer wavelength range, is strongly absorbed in virtually all materials. For that reason, extreme ultraviolet systems work by reflection rather than by transmission of light. Through the use of a series of mirrors, or lens elements, and a reflective element, or mask blank, coated with a non-reflective absorber mask pattern, the patterned actinic light is reflected onto a resist-coated semiconductor wafer.
The lens elements and mask blanks of extreme ultraviolet lithography systems are coated with reflective multilayer coatings of materials such as molybdenum and silicon. Reflection values of approximately 65% per lens element, or mask blank, have been obtained by using substrates that are coated with multilayer coatings that strongly reflect light essentially at a single wavelength within an extremely narrow ultraviolet bandpass; e.g., 12 to 14 nanometer bandpass for 13 nanometer ultraviolet light.
There are various classes of defects in semiconductor processing technology which cause problems in masks. For example, opaque defects are typically caused by particles on top of the multilayer coatings or mask pattern which absorb light when it should be reflected. Clear defects are typically caused by pinholes in the mask pattern on top of the multilayer coatings through which light is reflected when it should be absorbed. Further, the thickness and uniformity of multilayer coatings require manufacturing precision to not distort the image produced by the final mask.
In the past, mask blanks for deep ultraviolet lithography have generally been made of glass but silicon or ultra-low thermal expansion materials have been proposed as alternatives for extreme ultraviolet lithography. Whether the blank is of glass, ultra-low thermal expansion material, or silicon, the surface of the mask blank is made as smooth as possible by mechanical polishing with an abrasive. Another obstacle in mask blank creation includes scratches that are left behind in such a process are sometimes referred to as “scratch-dig” marks, and their depth and width depend upon the size of the particles in the abrasive used to polish the mask blank. For visible and deep ultraviolet lithography, these scratches are too small to cause phase defects in the pattern on the semiconductor wafer. However, for extreme ultraviolet lithography, scratch-dig marks are a significant problem because they will appear as phase defects.
Due to the short illumination wavelengths required for EUV lithography, the pattern masks used must be reflective masks instead of the transmissive masks used in current lithography. The reflective mask is made up of a precise stack of alternating thin layers of molybdenum and silicon, which creates a Bragg reflector or mirror. Because of the nature of the multilayer stack and the small feature size, any imperfections in the uniformity of the layers or the surface of the substrate on which the multilayer stack is deposited will be magnified and impact the final product. Imperfections on the scale of a few nanometers can show up as printable defects on the finished mask and need to be eliminated from the surface of the mask blank before deposition of the multilayer stack. Further, the thickness and uniformity of the deposited layers must meet very demanding specifications to not ruin the final completed mask.
In view of the need for the increasingly smaller feature size of electronic components, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.